PDF is the universal document format for fixed-layout content, invoices, contracts, scanned documents, e-books, forms. Created by Adobe in 1993 and made an open ISO standard in 2008, PDF preserves exact layout, fonts, and images across every device. Files can be searchable text, scanned images, or both. Most modern PDFs include a text layer that copy/paste and search work against.
How to open
Every modern browser opens PDFs natively. Acrobat Reader is free; macOS Preview, Windows Edge, and ChromeOS all open PDFs without extra software. For editing, Adobe Acrobat Pro or open-source PDFsam are common.